Encounter: Avery's Tower (chapter 10)
Select the Avery's Tower encounter from the main menu (chapter 10) and play it on explorer or light difficulty. There are two armored shotgun enemies here. First you need to do a jump takedown to take their helmet off. Then follow up with melee attacks. You are not allowed to shoot them at all (not even the helmet).
You can jump on them from the ruins.
Simply restart the checkpoint and repeat until you have 5 takedowns.